The New Zealand Postal Workers Union has waded in with concerns over Queenstown's missing mail and NZ Post's handling of the saga.

Last week, a fortnight after The Southland Times first asked, NZ Post confirmed the postie had only been contracted in those suburbs. Earlier, NZ Post said she had relieved in other areas short-term.

On Tuesday, Postal Workers Union president John Maynard sent a letter to NZ Post outlining the union's concerns and questioning why the company had not responded to questions repeatedly asked by The Southland Times.

"As a union we are very concerned and we are challenging the company.

"We are very upset with the lack of respect they've shown to customers making complaints . . . it appears they've been ignored or fobbed off."

Mr Maynard said the union and its members also held grave concerns about the damage the saga had done to the reputation of postal workers who had to deal with the public fallout and the company.

"This is a disaster. This is very bad and we are really concerned about it."

NZ Post logged 101 complaints from residents of Fernhill and Sunshine Bay between January 2010 and this month, increasing from one a month to 10 a month by August this year, when the company finally decided to act.

Mr Maynard said, given the size of the suburb and complaints in comparative areas with similar demographics, one complaint a month should have been enough to raise suspicions.

The union, which has about 1000 members, was also alarmed to learn NZ Post allowed contract staff to sub-contract work out which further undermined mail security, he said. It intended to raise all concerns with senior NZ Post staff at a meeting today.

NZ Post communications manager Michael Tull said contracting was common outside major cities but sub-contracting was less common. In Otago and Southland there were more than 40 delivery contractors, he said.

The Fernhill run had been contracted for at least 15 years. All contractors and sub-contractors had to be approved by the company.

"This approval process involves providing a range of personal details, agreeing to a background/security check and agreeing to submit to drug testing if and when this is requested by New Zealand Post," Mr Tull said.